1. Field of the Invention
The present invention relates to a radiation imaging apparatus for obtaining an image based on a phase shift of radiation and an image processing method for a radiation imaging apparatus.
2. Description Related to the Prior Art
Radiation, for example, X-rays have a characteristic that they attenuate depending on atomic number of an element constituting a substance and density and thickness of the substance. Due to this characteristic, the X-rays are used as a probe for inspecting inside of a subject in the fields of medical diagnoses and non-destructive inspections.
A common X-ray imaging apparatus comprises an X-ray source for emitting the X-rays and an X-ray image detector for detecting the X-rays. A subject is placed between the X-ray source and the X-ray image detector. The X-rays passed through the subject are imaged. To be more specific, the X-rays emitted from the X-ray source to the X-ray image detector are absorbed by the subject while passing through the subject and thereby attenuated. Then the X-rays are incident on the X-ray image detector. Hence, the X-ray image detector detects an image produced based on intensity changes of the X-rays caused by the subject.
The smaller the atomic number of the element, the lower the X-ray absorption power. Because the intensity changes of the X-rays caused by living soft tissue and soft matter are small, their images do not have sufficient contrast. For example, an articular cartilage of a human joint and its surrounding synovial fluid are composed mostly of water. A difference in X-ray absorption power between the cartilaginous part and the synovial fluid is small, resulting in poor contrast of the image.
Against this backdrop, recently, X-ray phase contrast imaging has been researched actively. The X-ray phase contrast imaging is a technique to image the phase shifts of the X-rays passing through the subject, based on the fact that the phase shifts are greater than the intensity changes. Thereby a high contrast image of the subject with low X-ray absorption power can be obtained.
An X-ray imaging apparatus for performing the above-described X-ray phase contrast imaging is suggested. In this X-ray imaging apparatus, first and second grids are arranged parallel with each other at a given interval, between an X-ray source and an X-ray image detector (for example, see Japanese Patent Laid-Open Publication No. 2008-200361). In this X-ray imaging apparatus, an X-ray image detector captures a moiré image of the X-rays emitted from the X-ray source and passed through the first and second grids. Thereby, a phase contrast image is obtained.
The X-ray imaging apparatus disclosed in the Japanese Patent Laid-Open Publication No. 2008-200361 utilizes a fringe scanning method. In the fringe scanning method, the second grid is moved intermittently relative to the first grid at predetermined regular intervals smaller than a grid pitch in a direction substantially perpendicular to a grid direction. The moiré images are obtained by capturing a moiré image each time the second grid halts. Based on the moiré images, an amount of the phase shift of the X-rays, caused by interaction with the subject, is detected and a differential phase image is produced. A phase contrast image is produced by performing an integrating process on the differential phase image.
The fringe scanning method requires a moving mechanism with high precision to move the first or second grid accurately at a pitch smaller than its grid pitch. This makes the apparatus complex and incurs high cost. The fringe scanning method requires to perform several image captures to produce the single phase contrast image. The motions of the subject and the grids during the series of image captures may degrade image quality of the differential phase image. The Japanese Patent Laid-Open Publication No. 2008-200361 suggests producing a differential phase image from a single moiré image obtained by a single image capture without moving the first and second grids, but a specific method is not disclosed.
U.S. Pat. No. 8,340,243 (corresponding to WO2010/050483) suggests a Fourier transform method. In this method, a moiré image is obtained by a single image capture without moving the first and second grids. Then, the moiré image is subjected to a series of processes: Fourier transform, extraction of a spectrum corresponding to a carrier frequency, and inverse Fourier transform. Thereby, a differential phase image is obtained.
In the Fourier transform method disclosed in U.S. Pat. No. 8,340,243, a peak position of a carrier frequency component cannot be obtained accurately because the distortion in the moiré fringes of the moiré image in a direction of a period or fringes, due to distortion, an arrangement error, or the like of the first and second grids, spreads the carrier frequency, and this causes a degradation of the image quality of the differential phase image.
In the Fourier transform method disclosed in the U.S. Pat. No. 8,340,243, the moiré image is transformed into a frequency space image by using the Fourier transform. Hence, the resolution in the frequency space decreases and the image quality of the differential phase image degrades when the X-ray image detector has a small number of pixels.
In the X-ray imaging apparatus disclosed in the U.S. Pat. No. 8,340,243, the positions of the first and second grids need to be adjusted with high precision and an X-ray image detector with a large number of pixels needs to be used so as to obtain uniform moiré fringes without distortion in period or direction to improve image quality of the differential phase image.